Cluster events can be used either inside eZ Publish (cache read/write, file handling...), but also through index_cluster.php in order to serve binary files. In the last case, the process should be as fast as possible and consume less resources as possible. Thus we should not include heavy dependencies on eZ Publish utility classes like eZDebug in order to log errors. This interface is to be implemented by classes dedicated to error logging, depending on context.